Method for pre-expanding and molding expandable thermoplastic polymer particles

ABSTRACT

An energy saving and material conserving method and apparatus are provided for expanding and molding expandable thermoplastic polymer particles. Especially significant is the ability to produce molded objects of generally uniform density in the higher density range in the order of 4-15 pounds per square inch, or greater, as well as lower density molded objects due to improved control of the particle expansion. 
     The particles are partially expanded in a vessel in a heated substantially dry atmosphere and agitated in a manner to provide uniform heating thereof. After an initial heating period to soften and partially expand the particles they are subjected to superatmospheric pressure to substantially prevent further expansion and are discharged and conveyed to a mold for further expansion therein. In certain instances, the particles are subjected to superatmospheric pressure within the vessel during a portion of the initial heating period to prevent premature particle expansion and loss of blowing agent within the particles. The temperature of the particles are continuously maintained above the boiling point of the blowing agent within said particles at all times subsequent to the partial expansion of the particles up to the final expansion and fusion of the particles in a mold.

This application is a continuation-in-part of U.S. application Ser. No.288,230, filed Sept. 11, 1972.

BACKGROUND OF THE INVENTION

This invention relates to the production of articles of expandedthermoplastic polymer material. Such articles are manufactured from thethermoplastic material which is provided in particulate form generallyreferred to as beads. The beads are made expandable during theirmanufacture by incorporating a so called blowing agent within. Becauseof their generally good flowing properties such expandable polymer beadscan be easily charged into a closed mold and accordingly lend themselvesreadily to the manufacture of these articles by expansion and fusion inclosed molds. Upon the application of heat the beads undergo a verygreat expansion; for example, they expand to about 30 to 40 times theiroriginal size. If expandable polymer beads were charged directly to themold for in situ expansion they would occupy very little space and wouldbe accumulated on the bottom of the mold. Consequently when heat isapplied to the mold, the beads, not being agitated, would not expanduniformly, the hotter particles expanding before the cooler particles.This would result in the premature fusion of the earlier softened andexpanded beads. The molded article would be poorly fused and containlarge voids and particles of widely varying densities. For these reasonsit is usually preferred to partially expand or "pre-expand" theparticles in an apparatus known as a pre-expander to approximately thedesired density of the finished article, fill the mold with thepre-expanded beads and further expand and fuse the pre-expanded beadstogether to form the finished article by applying heat to the particlesin the mold.

The pre-expansion is generally accomplished by heating the beads, whichcontain a blowing agent comprised of a gas or a composition which uponbeing heated to its boiling point produces a gas, to a temperature atwhich the bead walls are softened. The softened bead walls expand inresponse to the pressure exerted by the gaseous expanding agentcontained within them. The process may be carried out by either thebatch or continuous methods. A typical process for the manufacture ofpre-expanded polymeric particles is disclosed in U.S. Pat. No.3,023,175, issued on Feb. 27, 1962 to Rodman Jr.

While many of the foamed plastic products commonly in use such as icechests, floats, etc. are made of low density plastic foam there hasespecially in recent years developed a need for high density expandableor foam plastic items such as shoe platforms. Thus the inventive methodand apparatus disclosed herein is particularly capable of uniformlypre-expanding the commonly available expandable polymer beads to producehigh density (on the order of 8-15 pounds per square inch or greater)pre-expanded beads. The pre-expanded beads produced are particularlynotable for their uniform density of whatever the desired value and theyare dry and flowable and ready to be immediately charged into the moldfor molding of the manufactured article.

This is a great advantage over pre-expanders such as disclosed in theaforementioned U.S. Pat. No. 3,023,175 patent which uses steam to heatthe beads to pre-expand them in preparation for molding. Such a methodnot only results in wet beads that must be dried to make them flowablebut as described therein this method of pre-expansion results in therequirement for aging the beads for long periods of time in storagebins, or the like, for as long as twenty-four hours prior to their usein molding or irregular foaming will occur in the mold.

The method and apparatus of the present invention eliminates entirelythis long aging period and allows one to mold with the pre-expandedbeads immediately upon their leaving the pre-expander. Thus thesubstantial storage facilities and materials handling and conveyingapparatus required for aging the beads can all be eliminated with theresultant savings benefit.

As a result of the elimination of steam as the heating medium thepre-expanded beads leave the pre-expander of the present inventionsubstantially dry and in a flowable state so that it will not stick orjam and can be used to fill molds of any desired shape. Further sincethey may be immediately molded they will retain their heat from theirexposure in the pre-expander and it is found that relatively little heatin addition is required to be added in the mold to cause the beads tofurther expand therein and fuse together. Since the heating cycle can bethus shortened in the mold the cooling cycle is likewise shortened andthus an overall molding cycle time saving of up to about 60 percent maybe attained, as an example. The residence time of the beads in thepre-expander is likewise very short and thus substantial time and energysavings are made possible by the present invention.

SUMMARY OF THE INVENTION

To carry out the method and apparatus of the invention there is provideda vessel which is jacketed with a heating coil so that the insidethereof is heated by means of conduction and radiation through the wallsof the vessel. A predetermined amount of polymer bead containing ablowing agent is fed into the vessel which also contains an agitator.The agitator is especially designed to repeatedly wipe the beads againstthe heated walls of the vessel and then return them toward the centralportion thereof. Due to the heated condition of the beads the blowingagent within, as it reaches its boiling point at vaporizationtemperature, will vaporize and apply pressure against the heat softenedbead walls to expand the beads in this so called pre-expansion step.After a predetermined period of heating, the inside of the vessel issubjected to a superatmospheric pressure with a gaseous medium tosubstantially prevent further expansion of the beads. Thus due to thesubstantially uniform heating of all the particles within thepre-expander it is found that at the time of applying thesuperatmospheric pressure all such particles will have expanded tosubstantially the same extent and uniform density beads are obtained formolding the final articles. The density of the beads are determined bythe temperature they are exposed to within the vessel and the timeperiod of exposure before the superatmospheric pressure is appliedthereto. The ability to control the pre-expansions of the beads toenabling one to produce uniformly high density beads is an importantfeature of this invention as former pre-expanders have been mainlydirected toward producing lower density pre-expanded beads. For example,the pre-expander disclosed in U.S. Pat. No. 3,577,360 to Richard H.Immel is directed toward producing pre-expanded beads of ultra-lowdensity in the order of 0.4-0.75 pounds per cubic foot and exposes thebeads to a vacuum during pre-expansion to allow less restrainedexpansion of the beads. To make high density pre-expanded beads it isnecessary to control the expansion to a great degree which isaccomplished by the uniformity of the heating of the bead and the timelyapplication of superatmospheric pressure to the beads as disclosedherein. By control of temperature and length of the heating period it isalso possible to produce lower density pre-expanded beads, i.e., 1.25-3lbs per cubic foot (p.c.f.) in accordance with the invention and theuniformity of the density of the beads can also be maintained. Uniformdensity can mean savings to the producer since a higher density moldedproduct or portion of a product than desired will result in the use ofmore of the expensive polymer required to make any article. Uniformdensity also results in the better appearance of the finally moldedproduct as the visible cell size of the fused beads appears moreattractive. Further weakened portions of the article due to a lowerdensity area are avoided.

When the beads leave the vessel they are dry and flowable and still hotfrom being heated within the vessel. They may be fed directly into amold for molding of the final article or into an insulated storage binwhere several of the batches of the beads from the vessel are kept untiltransfer into the mold. Once in the mold further heat is applied to thebeads raising their temperature higher than in the pre-expansionthereof. The vaporized expanding agent and any gas, such as air, thatmay have permeated the bead wall will then further expand in volumeunder the higher temperature applying additional pressure against thesoftened bead wall to further enlarge the beads forcing them against oneanother within the mold. The heat softened polymeric bead walls willfuse together under these conditions filling up any space initiallybetween the beads into the mold when initially charged and upon coolinga one-piece cellular coherent article will be formed that may be removedfrom the mold which is then ready to be filled once more to repeat themolding cycle.

A critical feature of the invention is that the temperature of theexpandable polymer beads are continuously maintained above the boilingpoint or vaporization temperature of the expanding agent at all timessubsequent to the pre-expansion of the beads up to the final expansionand fusion of the beads in the mold. By maintaining the expanding agentin its gaseous state there can be no condensation, for example, of theexpanding agent to a liquid resulting in a vacuum inside the bead whichmay cause it to collapse due to atmospheric pressure thereon distortingany article molded from such beads. While the vacuum may be eventuallydissipated by allowing air, for example, to permeate the beads in a longaging period as previously described, it is the purpose of thisinvention to avoid such vacuum by maintaining the expanding agent in itsgaseous state at all time subsequent to the pre-expansion of theparticles up to the final expansion of the particles within the mold.

BRIEF DESCRIPTION OF THE DRAWING

The invention is illustrated in the attached sheet of drawing.

DESCRIPTION OF THE INVENTION

The term "thermoplastic polymer particles" is used generically in thepresent discussion and in the claims and as such includes all moldablethermoplastic polymer particles regardless of how they are made. Thus,the term includes polymer particles made by aqueous suspension typepolymerization, commonly known as beads or pearls comminuted particlessuch as those obtained by crushing and grinding slabs of masspolymerized thermoplastic material; and pellets obtained by extrudingthermoplastic polymeric materials and cutting the extrudate into smallparticles generally referred to as pellets.

The class of polymers found useful in the present invention are themoldable thermoplastic polymers, particularly the vinyl polymers.Included in this group are the vinyl aromatic polymerizable compoundssuch as polystyrene and polymers of derivatives of styrene; halogencontaining vinyl polymers, including polyvinylchloride andpolyvinylidene chloride; and acrylic polymers, such as polyethylacrylateand polymethylmethacrylate. Copolymers of the above with each other orwith other thermoplastic polymers can also be treated in the process ofthe invention. The preferred group of polymers for use in the inventionare the alkenyl aromatic polymers, particularly polystyrene.

Expanding or blowing agents suitable for use in the present inventionare those substances which are gaseous at atmospheric conditions ormaterials which are liquids or solids at atmospheric conditions butwhich will, upon being heated to a given temperature, produce a gaseoussubstance. The blowing agents are preferably substantially inert to thepolymer. Thus, the blowing agent may be a gas, such as propane orbutane, a low boiling inert liquid compound such as pentane, hexane,trichlorofluoromethane, etc., or a dry chemical compound such as sodiumbicarbonate. Combinations of the above blowing agents can also be usedin the invention. In general the blowing agent is present in an amountof about 2 to 15 percent and preferably about 3 to 10 percent based onthe total weight of the composition.

In carrying out the invention a predetermined amount of expandablethermoplastic polymer particles, hereinafter referred to as beads inaccordance with common usage, containing an expanding agent are insertedinto a closed vessel. The vessel is preferably cyclindrical in shape andincludes an agitator which is mounted on a shaft extending along theaxis of the cylindrical vessel. The agitator is continuously driven by amotor and provides a circulation of the beads in the vessel whereby theyare repeatedly wiped against the heated side walls of the vessel andthen returned to toward the center thereof whereupon they are againurged out toward the vessel walls to repeat the wiping action and so on.

The beads are heated while inside the vessel by means of their contactwith the heat conducting walls of the vessel and from the heat radiatingtherefrom. The amount of unexpanded beads fed to the vessel should bedetermined so that the charge when expanded to the desired density givesa volume not more than about 80-85% of the inside vessel volume. Thesource of heat for the side walls consists preferably of a heatingjacket comprising a coil of metal tubing encircling the vessel sidewalls through which steam under pressure is forced through. The steamconducting coil may be surrounded by an enclosing outer wall to preventheat loss. Alternatively electrically heated resistant coils could beused to heat the vessel walls or steam may be injected between an innervessel wall and an outer surrounding wall to form a so called steamjacket. The vessel walls are made of any good heat conducting metal suchas aluminum. Provision is made for sealing the vessel to produce aclosed vessel which can accommodate the build-up of superatmosphericpressure within. The superatmospheric pressure is provided for by meansof a compressed gas (preferably air) source which is connected byconveying line means to the vessel. A valve in the line may be manuallyand/or electrically operated to open and force the pressurized gas toflow into the vessel raising the gaseous pressure within acting on thebeads.

The vessel has inlet means for inserting a predetermined amount ofparticles within said vessel from a hopper and outlet means including aport or opening through which the particles may exit after thepre-expansion of the beads is completed. Any of the variously well knownvolumetric feeding devices such as shown may be utilized for chargingthe predetermined amount of expandable beads into the vessel. The outletmeans may comprise a known simple valve mechanism which may be openedeither mechanically or electrically as desired. The feeding device ispreferably also mechanically or electrically operated so that under highspeed manufacturing conditions electric timing devices may control theopening and closing of the inlet feeding and outlet means to preciselycontrol the residence time of the beads in the vessel. In practice theinside of the vessel is pressurized to control the bead expansion anormally short time (i.e., 10 seconds) before the outlet means to thevessel is opened so that the pressurized gas within while being relievedthrough the outlet means will carry the pre-expanded beads out of thevessel.

The heating means described also is preferably provided with temperaturecontrols to provide a means of controlling the amount of heat to whichthe beads are exposed to within the vessel.

As the pre-expanded polymer beads leave the vessel they may be feddirectly into molding means through conventional conveying means or maybe stored in an insulated storage hopper to await molding in a manner sothat they are maintained in their heated condition to the extent thatthe bead temperature is above the vaporization temperature of theblowing agent contained within the beads.

The atmosphere inside vessel surrounding the beads should be asubstantially dry gas, preferably air at substantially atmosphericpressure or slightly above due to the effect of heat thereon. The vesselmay be an entirely closed vessel, except of course for its inlet andoutlet ports when in operation or the vessel may be slightly vented butnot enough to preclude its pressurization to the necessary degree whendesired.

The temperature to which the expandable polymer beads are exposed to inthe vessel will depend upon the type of polymer being processed. As anexample, for polystyrene expandable beads, heating generally to atemperature of about 175°-180° F. will soften the polymer andtemperature of up to about 210°-220° F. can be used provided sufficientagitation is provided. The agitation described above to provide theuniform heating of the beads by the vessel walls further acts to preventthe beads from sticking to each other or agglomerating when the beadsare in a heated softened state within the vessel. For molding of thepolystyrene beads the beads are heated to a slightly higher temperatureof about 225° F.

The amount of time the beads remain within the vessel will depend upon anumber of variables including the type of polymer they are made of(i.e., the softening temperature range thereof) the amount of blowingagent incorporated within the beads, the temperature within the vesselto which the beads are exposed, the mode of agitation thereof and thedesired density of the pre-expanded bead to be produced for moldingpurposes.

The amount of superatmospheric pressure to which the beads are exposedwithin the vessel to control the density and uniformity of the beadswill likewise depend upon the variable mentioned above and will best bedetermined by gaining experience with the particular polymeric materialbeing used. Pressures inside the vessel on the order of about 40-50inches of mercury have been found to be usable.

As an additional feature of the present method and especially in theproduction of lower density pre-expanded beads (1.25-3 pcf) the vesselis pressurized using compressed air for a period immediately after beadsare inserted, the beads being maintained under super-atmosphericpressure during the initial stage of the heating period. It is believedthat the increased pressure on the beads will prevent loss of expandingagent by permeation from the beads during the initial heating stage. Theincreased pressure is then relieved by venting the vessel after apredetermined time period during which the beads are softened and thebeads are then allowed to expand under atmospheric pressure. If thevessel is a closed one a special selectively operable vent means isprovided to relieve the initially imposed superatmospheric pressure. Theretained expanding agent within the beads will then provide furtherinternal pressure from within the bead to expand them to a lower densitythan would be achieved otherwise. After the beads are expanded thedesired amount the vessel 10 is again pressurized as described above toachieve the desired uniform density before the beads are discharged. Themaintainance of the beads under superatmospheric pressure during theinitial stages of the heating period will also prevent prematureexpanding of certain of the beads which has been found to occurotherwise. The elimination of such premature bead expansion bypreventing expansion until all the beads are softened and can expandtogether when the intially applied superatmospheric pressure isrelieved, results in more uniformity of pre-expanded bead density thanpossible with previously known systems. Thus the application ofsuperatmospheric pressure in the initial stages of the heating period aswell as before discharge is found useful to make the more uniformdensity bead whether in the high density range (i.e. 8-15 pcf) or theaforementioned lower density range (i.e., 1.25-3 pcf).

The following example further illustrates the present invention.

EXAMPLE I

A horizontally mounted cylindrical vessel 24 inches long and 12 inchesin diameter of a prototype device was provided with a motor drivenaxially mounted agitator. A steam carrying coil for steam heating atpressures up to 100 psig is coiled around the vessel and a motor andbelt means for continuously driving the agitator is provided andsuitable inlet and outlet ports having electrically operated valves areconnected to an electric timing panel. A source of compressed air isconnected to the vessel and valve means electrically operated in presettimed sequence is provided to pressurize the vessel at the terminationof the heating cycle, the pressurized air discharging the pre-expandedbeads when the outlet means are opened. The pre-expanded beads arecarried by a conveying line directly to a standard molding press whereinthey are charged into a heated mold and molded in the standardcommercial manner to produce the finished fused cellular molded article.

A complete pre-expansion and molding cycle was run to demonstrate theadvantages gained in accordance with the objectives of the presentinvention. The polymer beads used were FOSTAFOAM expandable polystyrenebeads (3375) containing about 6% n-pentane by weight as the expandingagent. 106 grams of beads were charged to the vessel having thedescribed steam heating coil with steam at a pressure of 25 psigresulting in a temperature within the interior of the vessel of about215°-225° F. The beads were inside the vessel for a heating period of1.5 minutes when exposed to a 10 second period of superatmosphericpressure (on the order of about 48 inches of mercury) and thendischarged. Such beads were then immediately molded and the moldedarticle having a thickness of 1 inch was sectioned to determine thequality of fusion of the expanded beads in the final product.

The results of the test can be summarized as follows:

1. The pre-expanded beads were of very uniform size and density of 12pounds per cubic foot (pcf).

2. The molding cycle time saving was on the order of 50% resulting froma dramatic decrease in mold heating time (2 seconds vs. 14 seconds) ascompared to molding conventionally pre-expanded high density beads and avery substantial decrease in cooling time (1.5 minutes vs. 5 minutes).

3. The fusion of the beads in the 12 pounds per cubic foot densitymolded article was considered excellent and the article showed nodistortion from so called post expansion.

Similar results were obtained when the prototype device was used to moldarticles having densities of 7.5 pcf, 4.5 pcf and 1.5 pcf, the onlydifference being that the heating period required in the pre-expanderwas greater to produce the lower density articles (i.e., 7.5 pcfrequired 2.0 minutes, 4.5 required 2.7 minutes and 1.5 pcf required 4.5minutes).

Among important advantages of the invention, it is estimated that thecontrolled generally uniform density of the beads leaving thepre-expander results in a savings of about 5% of the polymer materialrequired. The greater density variations of the prior art pre-expandershave caused molders to prepare the pre-expanded beads at higher thanneeded densities so that they have a safety factor and are insured thatthe pre-expanded bead density will not fall below an acceptable minimum.With the present invention providing substantially more uniform densityof the pre-expanded beads, molders are able to reduce the safety factorand expand at lower target densities thus saving expensive polymermaterials. Since these polymers are generally derived from petroleum anysavings in this area is of course important in the present petroleumsupply sensitive world climate.

The saving in mold cycle time attainable in accordance with the abovedescription of the present invention have further petroleum and otherenergy source saving ramifications. Heating time period reductionspossible, as explained above, as well as cooling cycle reductionsrequiring the pumping of cooling fluid, and the eliminating of aging thepre-expanded beads and the conveying and reheating requirementattributable thereto, all result in significant energy requirementreductions. It has been estimated that overall utility cost savings dueto these economics may amount to as much as 75% over conventionalmoldings.

Thus it has been demonstrated that the present inventive method andapparatus provides many advantages resulting in both technical andeconomic benefit to the expandable polymer molding industry includingimport energy conserving reductions in molding cycle requirements whichalso serves the national interest.

I claim:
 1. A method of mold expandable thermoplastic polymer particlescontaining a blowing agent comprising:(a) inserting a measured amount ofthe particles into a vessel; (b) heating the particles in asubstantially dry atmosphere within said vessel to a temperature highenough to soften the polymer particles and volatize the blowing agent,the heat applied by conduction through the walls of the vessel andradiation therefrom and causing partial expansion of the particles; (c)agitating the particles within said vessel at all times during theheating thereof in a manner so that the particles are repeatedly wipedby an agitator within said vessel against the walls thereof and thenreturned toward the center of said vessel; (d) subjecting the particleswhile in their heat softened state to a super atmospheric pressure witha substantially dry gaseous medium after a predetermined period ofheating in said vessel the pressure being adequate to substantiallyprevent further expansion of the particles to obtain the desiredgenerally uniform density thereof; (e) removing the particles from saidvessel with the aid of the gaseous medium under pressure which carriessaid particles out of the closed vessel when an exit opening is providedfor the particles. (f) conveying the heated partially expanded particlesto a mold and filling the mold therewith and; (g) heating the particlesin the mold to effect the further expansion and fusion of the particlesinto the desired molded article; The temperature of the particles beingcontinuously maintained above the boiling-point of the blowing agentwithin said particles at all time subsequent to the partial expansion ofthe particles in said vessel up to the final expansion and fusion of theparticles within the mold.
 2. The method of claim 1 wherein theparticles are exposed to substantially dry air at generally atmosphericpressure during the heating period within the vessel except when theparticles are exposed to superatmospheric pressure before the dischargethereof from said vessel.
 3. The method of claim 1 wherein the particlesare exposed to substantially dry air at generally atmospheric pressureduring the heating period within the vessel except when the particlesare exposed to superatmospheric pressure both during a time period soonafter insertion into the vessel in order to prevent premature beadexpansion and undesirable loss of blowing agent and in a time periodjust before discharge of the beads from the vessel in order to controlbead expansion to achieve the desired generally uniform bead densitydesired.
 4. The method of claim 1 for molding expandable thermoplasticpolymer particles wherein during the molding cycle subsequent to thepartial expansion of said particles in said vessel the temperature ofthe particles is maintained only slightly lower than that required toaffect said further expansion and fusion of the particles within themold.
 5. The method of molding of claim 1 wherein the expandablethermoplastic particles are of polystyrene containing about 6% by weightof pentane as the blowing agent.
 6. A method of partially expandingexpandable thermoplastic polymer particles containing a blowing agent,said particles being capable of further expansion, comprising:(a)inserting a measured amount of the particles into a vessel; (b) heatingthe particles in a substantially dry atmosphere within said vessel to atemperature high enough to soften the polymer particles and to volatizethe blowing agent, the heat applied by conduction through the walls ofthe vessel and radiation therefrom and causing partial expansion of theparticles; (c) agitating the particles within said vessel at all timesduring the heating thereof in a manner so that the particles arerepeatedly wiped by an agitator within said vessel against the wallsthereof and then returned toward the center of said vessel; (d)subjecting the particles while in their heat softened state to a superatmospheric pressure with a substantially dry gaseous medium after apredetermined period of heating in said vessel the pressure beingadequate to substantially prevent further expansion of the particles toobtain the desired generally uniform density thereof; and (e) removingthe particles from said vessel with the aid of the gaseous medium underpressure which carries said particles out of the closed vessel when anexit opening is provided for the particles. The temperature of theparticles being continuously maintained above the boiling point of theblowing agent within said particles at all time subsequent to thepartial expansion of the particles in said vessel.
 7. The method ofclaim 6 wherein the particles are exposed to substantially dry air atgenerally atmospheric pressure during the heating period within thevessel except when the particles are exposed to superatmosphericpressure before the discharge thereof from said vessel.
 8. The method ofclaim 6 wherein the particles are exposed to substantially dry air atgenerally atmospheric pressure during the heating period within thevessel except when the particles are exposed to superatmosphericpressure both during a time period soon after insertion into the vesselin order to prevent premature bead expansion and undesirable loss ofblowing agent and in a time period just before discharge of the beadsfrom the vessel in order to control bead expansion to achieve thedesired generally uniform bead density desired.
 9. The method of moldingof claim 6 wherein the expandable thermoplastic particles are ofpolystyrene containing about 6% by weight of pentane as the blowingagent.